This invention relates to print cartridges for inkjet printers and more specifically to the expulsion of ink from an inkjet printer printhead.
Inkjet printing mechanisms use pens that shoot droplets of colorant onto a printable surface to generate an image. Such mechanisms may be used in a wide variety of applications, including computer printers, plotters, copiers, and facsimile machines. For convenience, the concepts of the invention are discussed in the context of a printer. An inkjet printer typically includes a printhead having a plurality of independently addressable firing devices. Each firing device includes a firing chamber connected to a common ink source, an ink propulsion device, and an ink expulsion nozzle. The ink propulsion device within the firing chamber provides the impetus for expelling ink droplets through the nozzles.
In thermal inkjet pens, the ink propulsion device is a resistor that provides sufficient heat to rapidly vaporize a small portion of ink within the firing chamber. The bubble expansion provides for the displacement of a droplet of liquid ink from the nozzle. The heat to which the ink is exposed in a thermal ink jet pen prevents the use of thermally unstable ink formulations that might otherwise provide desirable performance and value. Therefore, the available ink options are reduced to those that are not adversely affected by varying temperatures.
Conventional piezoelectric inkjet pens avoid the disadvantages of thermally stressing the ink by using a piezoelectric transducer in each firing chamber. The firing chamber dimensionally contracts in response to the application of a voltage to provide the displacement to expel a droplet of ink having a volume limited to the volume change of the piezoelectric material. Because of the very low displacement or equivalent strains (&lt;1%) of piezoelectric material, conventional piezoelectric transducers have limited volume displacement capability requiring relatively large crystals thereby reducing packing density. Furthermore, piezoelectric transducers are susceptible to degradation by direct exposure to some inks that might otherwise be desirably employed, and have other disadvantages related to limited miniaturization, cost, and reliability.
With the invention as described hereinafter, an ink expulsion actuator is manufacturable that has increased ink flexibility; is a more predictable and repeatable actuator by the elimination of thermal cycling used in conventional inkjet propulsion systems which eliminates unpredictable ink nucleation variations; and, allows discrete control of ink drop size through the control of voltage due to the increased displacement or strain (up to 30%) of electrostrictive polymer actuators over piezoelectric devices.